The 3rd Generation Partnership Project (3GPP) is presently defining various communication protocols for the emerging next generation, LTE (Long Term Evolution) Advanced cellular telecommunication standard, which employs an air interface formally referred to as E-UTRA (Evolved-UMTS Terrestrial Radio Access).
LTE standards employ OFDM transmission between an eNode-B base station and multiple user equipment devices (UEs) in which an available spectrum is divided into numerous relatively narrow bandwidth carriers, each on a different frequency. An example of a transmission time interval is shown in FIGS. 1A and 1B, to which reference is now made. Multiple carriers are located on the Y axis and time is shown on the X axis. In FIGS. 1A and 1B, three carriers within the TTI (transmission time interval) are allocated, one each to three user equipment devices, UE1, UE2 and UEN. In E-UTRA terminology, each carrier is designated as a physical resource block or PRB 20. As can be seen by the grid, each PRB 20 is separated into time-frequency bins 22, over which a portion of a message is sent.
FIG. 1A illustrates an example of a first TTI, TTI1 and FIG. 1B illustrates an example of a second, TTI, TTI2. During each time interval TTI, the base station allocates PRBs 20 to the UEs and to the control channel. The allocations of PRBs 20 may change from TTI to TTI. Thus, in FIGS. 1A and 1B, the three UEs are allocated to different PRBs 20. In FIGS. 1A and 1B 15 PRBs 20 are shown. The 15 PRBs are labeled 20A-20O and PRBs 20D, 20G and 20K and are allocated to UEN, UE1 and UE2, respectively. In FIG. 1B, only the allocated PRBs 20 are labeled. Thus, PRB 20B is allocated to UE1, PRB 20J is allocated to UE2 and PRB 20O is allocated to UEN.
A downlink control channel (Control) from the base station is also transmitted in every TTI. However, as can be seen in FIGS. 1A and 1B, the first column (or first few columns) of the transmission time interval is allocated to the control channel. This allocation corresponds to a transmission over all frequencies. The control channel carries the PRB allocations to the UEs in the current TTI. It also may pass common messages from the base station to the UEs.
FIG. 1C illustrates an example of a control structure. Within the transmission are pointers for each UE, pointing to the PRBs allocated to it for the current TTI. In the illustration of FIG. 1C, the pointers are listed in order, from UE1 to UEN. However, this need not be the case. Each pointer is encoded with the ID number of each UE and thus, each UE, upon reading the control allocation, need only decode each pointer with its ID number. All pointers which successfully decode are pointers for that UE.
In order to support high data throughput, it is important for a network to perform effective scheduling and data transmission over the multiple carriers. Data transmission from the eNode-B base station to the UE may utilize, for example, different modulation and coding schemes (MCSs) as a function of channel quality. Other transmission parameters may also be affected by channel quality.
Estimation of channel quality typically involves separate power measurements of signals and of the interference or noise at any given moment. The UEs transmit these measurements to the base station, typically in the form of a channel quality indicator (CQI) signal, and the base station may transmit instructions such that throughput is maximized, taking into account channel quality. For example, more optimal MCS, antenna arrangements and the like may be used. However in some OFDM networks, such as LTE networks, interference experienced by any particular UE may differ among the time/frequency bins and may be affected, among other things, by the network traffic load, signals from neighboring cells and the like. All of these influences may impact on interference measurement and related CQI reports.
One conventional approach for estimating the interference is based on measurement of the noise at specified RS (reference signal) bins during which the base station transmits a reference signal, that is defined by known symbols, for use in channel estimation. Such an approach has the disadvantage that the interference at RS bins may be systematically different than interference at pertinent data bins.
According to another conventional approach, multiple sparse “holes”, shown by hashing in FIGS. 1A and 1B, are distributed at fixed locations throughout the time-frequency space in a TTI. Instead of transmitting RS symbols, the base station does not transmit anything in these specified holes. Since the holes are not allocated for signal transmission, UEs can readily estimate the interference in these holes, which corresponds to noise. This conventional approach, however, reduces available transmission capacity in the time/frequency bins allocated to transmitting data. In FIGS. 1A and 1B, the holes may be found in PRBs 20A, 20D, 20G, 20J and 20M. In FIG. 1A, this reduces the available transmission capacity of UEN and UE1. In FIG. 1B, the holes affect UE2 and reduce its transmission capacity.